docs: def get_compounds(identifier, namespace='cid', searchtype=None, as_dataframe=False, **kwargs):
      """Retrieve the specified compound records from PubChem.
  
      :param identifier: The compound identifier to use as a search query.
      :param namespace: (optional) The identifier type, one of cid, name, smiles, sdf, inchi, inchikey or formula.
      :param searchtype: (optional) The advanced search type, one of substructure, superstructure or similarity.
      :param as_dataframe: (optional) Automatically extract the :class:`~pubchempy.Compound` properties into a pandas
                           :class:`~pandas.DataFrame` and return that.
      """
      results = get_json(identifier, namespace, searchtype=searchtype, **kwargs)
      compounds = [Compound(r) for r in results['PC_Compounds']] if results else []
      if as_dataframe:
          return compounds_to_frame(compounds)
      return compounds